1. Field of the Invention
The present invention relates to an image processing apparatus and a method of adjusting the same for obtaining a reproduced image.
2. Related Background Art
A method of reproducing a half-tone image by using a laser beam printer or the like has been proposed by the same applicant of this application, wherein inputted digital image information is converted into analog image information which is compared with a periodical analog pattern signal such as a triangular signal to obtain pulse-width-modulated and binarized image information.
FIG. 4 shows a circuit diagram embodying this method. In the figure, an inputted 8-bit digital video signal VD.sub.0 to VD.sub.7 is latched by a latch circuit 1 in synchro with a video clock signal 1/2CLK. The output VD.sub.0 to VD.sub.7 of the latch circuit 1 is inputted to a gain selection means (ROM) 16 at its address bits A.sub.0 to A.sub.7 to read a digital video signal VR.sub.0 to VR.sub.7 whose amplitude and bias level have been changed from those of the digital video signal VR.sub.0 to VR.sub.7. The digital video signal VR.sub.0 to VR.sub.7 is converted into an analog video signal VA by a D/A converter 2 which analog video signal VA is converted into a desired voltage level by a resistor 3 to apply it to one input terminal of an analog comparator 4. A master clock signal CLK is divided by n at a frequency divider 6 to provide a clock signal 1/n CLK which is then divided by 2 at a J-K flip-flop 8 to provide a pattern clock signal PCLK having a duty ratio of 50%. Thus, the pattern clock signal PCLK has a period n times as long as that of the video clock signal 1/2 CLK. The pattern clock signal PCLK is then inputted via a buffer 9 to an integration circuit composed of a resistor 10 and a capacitor 11 to provide a triangular signal (analog pattern signal) SAW having the same period as that of the pattern clock signal PCLK. The bias component of the triangular signal SAW is adjusted by a capacitor 12 and a resistor circuit 13-1, 13-2 and applied to the other terminal of the comparator 4 via a protection resistor 14 and a buffer amplifier 15. The comparator 4 compares the analog video signal VA and the triangular signal SAW so that the analog video signal SAW is pulse-width-modulated in accordance with its density to thereby output a pulse-width modulated signal Pw from the comparator 4. An unrepresented laser driver operates based on the pulse-width-modulated signal to modulate a recording beam. To achieve a good tone property with this circuit, it is desirable to have a relation, as shown in FIG. 6, between the levels of the analog video signal VA and the triangular signal SAW. In particular, the relation is that the maximum level VAmax (e.g., black level) of the analog video signal VA coincides with the peak level of the triangular signal SAW, and that the minimum level VAmin (e.g., white level) of the analog video signal VA coincides with the bottom level of the triangular signal SAW. Such a relation ensures a maximum resolution and a full scale linearity.
Apart from the above, images reproduced with such a circuit have various image tones (image characteristic or property). For instance, as to a character image, reproducing pixels changing from white to black or vice versa with high fidelity is more important than reproducing half-tone, while as to a picture image, reproducing half-tone is more important. Therefore, the circuit of FIG. 4 is arranged to select the period of the pattern clock signal PCLK depending on which tone property is to be reproduced emphatically. In particular, the frequency divider 6 can change its division ratio, e.g., 1 to n in accordance with a period selection signal SEL. During reproducing a character image, the division ratio is set at 1 for example. As a result, one pixel of the inputted digital video signal is pulse-width-modulated using one triangular signal SAW so that pixels changing from white to black or vice versa can be reproduced with high fidelity. On the other hand, during reproducing a picture image, the division ratio is set at n for example. As a result, n pixels of the inputted digital video signal are pulse-width-modulated using one triangular signal SAW so that an image of smooth tone property can be reproduced. With this circuit, however, when the division ratio is switched, not only the period of the triangular signal SAW but also its amplitude and bias are changed, to thereby become unable to satisfy the relation of FIG. 4. In view of this, ROM 16 has been incorporated to change the amplitude and bias of the video data VA in response to changing the period. In particular, ROM 16 stores image data conversion tables TABL1 to TABL4 which define four combinations of gain G and bias B, as exemplarily shown in FIG. 5. One of the combinations is selected based on the period selection signal SEL. The table selection is made when the frequency divider 6 changes its division ratio. Thus, the gain and bias to be used for the inputted image information is changed when the period of the periodical pattern signal SAW is changed, thereby always making the peak and bottom values of the triangular signal SAW respectively coincide with the maximum and minimum values VAmax and VAmin of the image information VA subjected to gain conversion (or .gamma. conversion). For example, if a designated period T.sub.1 is short, gain G.sub.1 is set small and bias B.sub.1 is set large, whereas if a designated period T.sub.3 is long, gain G.sub.3 is set large and bias B.sub.3 is set small. To perform such function, ROM 16 has a capacity of 1k bytes, wherein bits D.sub.0 to D.sub.7 of the digital video signal are inputted to lower 8 bits A.sub.0 to A.sub.7 of the address terminals, and the period selection signal SEL is applied to upper 2 bits A.sub.8 and A.sub.9. Thus, it is possible to change the conversion table for the analog video signal VA simultaneously with switching the period of the triangular signal SAW.
However, values of the elements (resistor 10, capacitor 11 and so on) constituting the triangular signal generating circuit may disperse in practice, so the smplitude and bias of the triangular signal SAW may also disperse. Consequently, provision of only a single table for each designated period is not sufficient for compensating a small deviation caused by such dispersion. If tables were to be provided in one-to-one correspondence with all possible triangular signals SAW generated from such dispersion of the circuit elements, the number of tables becomes extraordinarily large and is not applicable to practical use.